The Influence of Magma Ocean Processes on the Present-day Inventory of Deep Earth Carbon
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The Influence of Magma Ocean Processes on the Present-day Inventory of Deep Earth Carbon
Rajdeep Dasgupta
CIDER post-AGU workshop December 10, 2011
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Depleted mantle
50-200 ppm CO2
Enriched mantle
up to 1000 ppm CO2
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What is the influence of the modern plate tectonic cycles on the deep Earth carbon inventory and distribution?
Is the Earth’s present-day mantle carbon budget shaped by early Earth processes?
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Behavior of C during early Earth differentiation (metal-silicate equilibration and reduced magma ocean)?
In what form and capacity the magma ocean retained carbon?
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Experimental simulation of metal-silicate equilibration in a magma ocean environment
Experiments: 1 – 7 GPa; 1500 – 2000 °CDevice: Piston cylinder (Rice) and Multi-anvil (Lamont-Doherty Earth Observatory)
Starting Mix: Basalt/komatiite/peridotite + Fe-Ni + C ± S
Han Chi
FeO (silicate melt) = Fe (alloy melt) + 1/2O2
2log(aFeO/aFe) = IW
2 GPa, 1625 °C
fO2 = IW-1 to IW-2
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Carbon Solubility at Graphite Saturation, CCGC (fO2 ~ IW-1.6)
Chi et al. (in progress)
Silicate Melt
Fe-5%Ni Melt
CCGC in basaltic melt is small and increases with increasing T and decreases with increasing P
CCGC in Fe-rich metallic melt is significant
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Chi et al. (in progress)
Carbon partitioning during metal-silicate equilibration
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Depleted (MORB) mantle – 50-200 ppm CO2; Enriched (OIB) mantle – up to 1000 ppm CO2
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Carbon partitioning into segregating metallic liquid
1. Trapped metallic liquid, metal carbide, and metal alloy in crystallizing magma ocean
2. C-ingassing from early atmosphere
C in atmosphere
1
2
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Closing Remarks
• Carbon dissolution in reduced magma ocean was primarily in the form of neutral or hydrogenated species AND solubility was likely low (≤100 ppm C)
• Most of early Earth carbon was partitioned into metallic Fe-Ni liquid and subsequently sequestered in the metallic core OR trapped as interstitial alloy or carbide phase
• Convective overturn of lower mantle materials (with interstitial carbide or alloy) may have supplied a sizeable portion of the initial carbon budget of the mantle
• Depth dependence of carbon concentration in reduced basaltic magma at graphite/diamond saturation may also have facilitated ingassing of carbon from the early atmosphere
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How much carbon was dissolved in the silicate magma ocean during core segregation?
C measurement in silicate glasses using 12C/30Si versus basaltic melt CO2 calibration using ion probe
Chi et al. (in progress)
Cameca IMS 1280 ion probe at WHOI
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Graphite
Fe-Ni metallic melt
Silicate melt
SIMS pits
1 mm
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2 GPa, 1625 °C, Fe-Ni melt saturated basaltic melt in a graphite capsule
At fO2 of IW-1 to IW-2, CO2 solubility (in graphite and metal saturated conditions) in basaltic silicate melt is minimal
Chi et al. (in progress)
Typical carbon bearing basalt FTIR spectra at oxidized (~IW+5) conditions